Should I cover my field of Canadian Bluejoint grass to get rid of bittersweet and roses?

Question:

I have a field of Canadian Bluejoint grass, which has been taken over by bittersweet and roses. I usually cut it all down each year, but I would like to return the field to more of the Canadian Bluejoint grass. I heard I can solarize it by covering it with black material. I would like more specific information and material suggestions.

Answer:

Jonathan Foster, Home Horticulture Outreach Professional

So, a couple of things we should go over. The first thing I would mention is that any physical strategy like this to remove the unwanted plants will probably affect the blue joint, as well. It may still wind up being your approach, but just know that it may require replanting the grass afterward. Both rose and bittersweet can be difficult to eradicate–in particular, mowing the bittersweet often boosts root growth, so you will definitely want to include a secondary method to go with the mowing for effective control. And depending on the size of the field, this may be quite a large job.

Solarization and the closely related occultation are explained nicely in this resource from the Univ of MN Extension. They can be effective techniques for killing weed seeds, low level unwanted plant infestations, and many soil borne pests and pathogens. The principles are 1) high heat from a greenhouse effect kills seeds and many germs directly, and 2) the heat and humidity encourage existing plant growth, but then prevent water and/or light from reaching the plants, killing them through dehydration. However, it probably won’t be quite the proper approach for a field with established bittersweet and roses, as I worry their vigorous growth may just push through the barriers.

If we’re really trying to regroup, get rid of the invasives, and start over, I would recommend doing your standard mowing or possibly even tilling the field (normally this would be tricky because there is probably a huge supply of rose and bittersweet seed in your soil seed bank there, and disturbing the soil tends to bring those to germination depth, but you’re trying to just remove bulk so you can move to the next step) and then pursuing a smothering strategy. This is similar to solarization/occultation, but uses a more substantial barrier (you can use thick black plastic, but tarps are also often used) and then covers it with a layer of mulch to double down on preventing growth from below and light from above. 

Smothering strategies can be effective, but they require persistence and attention, and we generally advise them for smaller spaces. Established rose and bittersweet will try to push through the barrier, as well as seek the edges of the barrier to grow out from under. With the mulch on top, it will be easier to spot shoots coming through so they can be cut down, but it will take several seasons of diligence before you can consider pulling it all back off to evaluate.

Those are the basics, but I still worry this may be ineffective for a large field simply because of the scope of the project. While we generally discourage herbicide use unless it’s absolutely necessary and other options haven’t worked, but it might be worth consulting with a ME-licensed pesticide applicator to see if treating the field would be a better option.

Good luck and happy gardening.